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Report No.

Effects of $$gamma$$-ray irradiation on the high temperature oxidation of stainless steel in supercritical water

Nakahara, Yukio ; Yamamoto, Masahiro ; Kato, Chiaki   ; Kiuchi, Kiyoshi; Karasawa, Hidetoshi*; Katsumura, Yosuke*

Corrosion experiments of stainless steel (SS) in supercritical water (SCW) have done with a flowing SCW loop including a $$gamma$$-ray irradiated autoclave. Three types of commercial-grade austenitic SS, Type 304L, Type 316L, and Type 310S, were examined. The condition of SCW was adjusted to the core environment of a SCW reactor system. Test samples of the SS were immersed in deaerated pure SCW (25 MPa, 550 $$^{circ}$$C). Feed water was circulated at a flow rate of 4 L h$$^{-1}$$. Absorbed dose rates near the surface of the test samples in SCW were estimated at 5-15 kGy h$$^{-1}$$. The time dependence of net weight gains followed parabolic rate law except $$gamma$$-ray irradiated Type 316L SS. The net weight gains of $$gamma$$-ray irradiated Type 304L SS and Type 310S SS were larger than those of the non-irradiated. Rate constants of parabolic rate law in net weight gains of Type 304L SS and Type 310S SS were decreased as the $$gamma$$-ray exposure rate was increased. The net weight gain of $$gamma$$-ray irradiated Type 316L SS was fluctuating and decreasing widely, because the exfoliation of the surface oxide occurred. On the surface of all the samples, the formation of two-layer oxide film in which the outer layer was porous iron oxide and the inner layer was dense iron-chromium-nickel oxide occurred. Iron oxide in the outer layer was magnetite (Fe$$_{3}$$O$$_{4}$$) with non-irradiated samples. Hematite ($$alpha$$-Fe$$_{2}$$O$$_{3}$$) was additionally formed in the outer layer with $$gamma$$-ray irradiated samples. A chromium-rich part in the inner layer was formed along to the oxide/metal interface of $$gamma$$-ray irradiated Type 304L SS.



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